In the wireless communication system, the channel environments vary with time. The adaptive modulation system is one of techniques that enhance the transmission rate in such channel environments. The adaptive modulation scheme controls the modulation scheme in accordance with channel environments. For example, if the channel environment is judged to be poor, the data is transmitted using a modulation scheme which is highly reliable. If, on the other hand, the transmission environment is satisfactory, the data is transmitted using a modulation scheme that can hold a large amount of information. If a base station apparatus and a terminal apparatus included in a wireless communication system communicate using a TDD (Time Division Duplex) scheme, a slot for the uplink and that for the downlink are generally transmitted alternately on a regular basis.
If the reversibility of a channel holds in the TDD scheme, the base station apparatus receives the slots and detects a C/N (Carrier to Noise Ratio), delay spread, RSSIs (Received Signal Strength Indicators) or the like of the channel, from information, on receiving level, such as received baseband signals so as to estimate a channel environment at the next transmission timing. In accordance with this estimation result, the base station apparatus selects a modulation scheme to be used for transmission (see Patent Document 1, for instance).
[Patent Document 1]
(1) Japanese Patent Application Laid-Open No. 2002-290246.